The use of adhesive structural bonding has spread rapidly throughout the aerospace industry. Its wide acceptance is attributed, on one hand, to unique strength properties imparted to joined structures by the adhesives and, on the other hand, to the design flexibility and production simplification it provides. Adhesive bonding is, in many instances, replacing conventional techniques, such as welding, brazing, and mechanical fastening.

A major problem encountered in the effective application of structural adhesives, however, is determining the reliability of the bonded assembly. Up to the present time, the quality of a bonded structure has been difficult to establish without actually testing the structure to destruction. Test specimens bonded under the same process and at the same time, destructively tested, cannot provide complete quality assurance. The test specimens can exhibit and guarantee only that bonded assemblies are properly processed, that is, that proper surface preparation and cure treatment were used. The test coupons can, therefore, indicate the adhesive quality of a joint, but not the cohesive quality of the bondline.

A nondestructive testing procedure that provides an indication of the cohesive quality of a joint has been studied, developed, and applied to the production-line testing of bonded structures at Hughes Aircraft Co.

This paper briefly describes the background for this study, the procedure developed, and the technique used to generate the measurement criteria.